(1) Field of the Invention
The present invention relates to an information recording medium, a playback method thereof, and a recording method thereof, in particular, to a technology for realizing special playback such as fast-forward/rewind. Here, the information recording medium is a BD-ROM or the like having recorded thereon contents such as video and audio, with copyright protection.
(2) Description of the Related Art
Special playback functions such as fast-forward/rewind are indispensable functions for playing back an AV stream recorded on a BD-ROM, DVD, or the like.
In general, a player can perform special functions such as fast-forward/rewind by executing reading of a disc, decoding of the read data, and the like at a speed faster than a normal playback speed, and then playing it back. However, this method places burden on the player, causing the player to be unable to perform processing at the speed required for high-speed playback such as 10× for fast-forward/rewind.
Accordingly, in order to realize special playback such as fast-forward/rewind, the player needs to read and decode part of the AV stream while making temporal skips. For example, the player may select intra-frame encoded images (hereinafter, referred to as “I-pictures”) in video included in an AV stream in accordance with the speed of fast-forward/rewind, and performs reading and decoding. However, an AV stream on a BD-ROM or DVD contains various streams such as video, audio, and subtitles multiplexed into the AV stream. Besides, pictures vary in size in a case where the video is encoded using variable length encoding. Accordingly, identifying an I-picture pertaining to a desired time requires analysis processing of the AV stream, thereby consuming a large amount of processing time.
Thus, BD-ROMs use entry maps to realize fast-forward/rewind. An entry map is composed of a list of multiple entry points each indicating an access position in an AV stream. Each entry point is information in which a file position within the AV stream corresponds to a playback time of data located at the file position. For example, an entry map including entry points each composed of a starting file position of an I-picture included in an AV stream on an information recording medium and the presentation time of the I-picture allows the playback apparatus to identify the position, of the I-picture, corresponding to the playback time without analyzing the AV stream, by referring to the entry map. As a result, the playback apparatus is able to perform partial playback of the AV stream, thereby realizing efficient fast-forward/rewind without burden. Patent Document 1 discloses a data structure and a creation method of an entry map for identifying positions of I-pictures.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2000-228656
In manufacturing BD-ROM playback apparatuses which handle HDTV contents, demand for copyright protection at the level of TS packets constituting the AV stream is high. However, in order to realize copyright protection using such as special encryption and transformation at the TS packet level, it is required to perform decryption processing, restoration processing or the like in real time at the uppermost stream when reading from the BD-ROM. Especially if the AV stream is provided for a high-speed special playback such as over 10×, such real time processing will incur a large burden.
In addition, multiple video streams may be multiplexed into an AV stream in a BD-ROM to realize Picture-in-Picture playback or 3D playback.
If TS packets constituting two video streams in an AV stream are transformed and the AV stream is provided for a high-speed special playback such as 10×, simultaneous restoration of the transformed TS packets of these multiple video streams need to be realized at 10×. If the BD-ROM playback apparatus is to be designed assuming such a worst-case scenario, it will inevitably require addition of special hardware, increase in operation clock speed, and the like. This will necessitate further modification and improvement of the present BD-ROM players, which goes against the idea of encouraging broad use of the playback apparatuses through standardization.